Characterization of tunable longwave infrared filters using quantum cascade lasers

We describe performance characterization of spectrally tunable nano-engineered filters operating in the longwave infrared (LWIR) from 8 to 12 micron using quantum cascade lasers (QCLs) tunable over the full spectral range. The filter design is based on using the guided mode resonance (GMR) phenomenon. The device structure consists of a subwavelength dielectric grating on top of a planar waveguide using high index dielectric transparent materials, i.e., germanium (Ge) with a refractive index of 4.0 and zinc selenide (ZnSe) with refractive index of 2.4. The filters are designed to reflect the incident broadband light at one (or more) narrow spectral band while fully transmitting the rest of the light. The tuning of the reflection wavelength is achieved by changing the angle of incidence of light by mechanically tilting the filter. Filters based on one dimensional (1D) gratings are polarization dependent and those based on two dimensional (2D) gratings are close to polarization independent. To design the filter with a strong narrow band reflectance, we used the rigorous coupled wave (RCW) algorithm to simulate the filter. Here we will describe design and characterization of prototype filters with 1D grating. Anti-reflection coatings were applied to improve transmission over the entire spectral region. Our experimental setup consists of a QCL system operating at room temperature, nanoengineered filter and an uncooled broadband sensor. We will present the filter design, detailed characterization experiment and compare the theoretical and experimental results.